1. Field of the Invention
The present invention relates to a power transmission device for a four-wheel drive vehicle comprising main drive shafts to which the torque of an engine is directly transmitted and sub-drive wheels to which a part of the torque of the engine is indirectly transmitted via a drive shaft, a hydraulic clutch and a driven shaft.
2. Description of the Related Art
The power transmission device for a four-wheel drive vehicle is known through JP-A-3-224830. This power transmission device is constructed such that a hydraulic pressure closed circuit is constituted by a first hydraulic clutch adapted to interlock with the front wheels for rotation and a second hydraulic pump adapted to interlock with the rear wheels for rotation and that the hydraulic clutch is adapted to lock under hydraulic pressure generated in the hydraulic pressure closed circuit by a differential rotation between the front and rear wheels, whereby the part of the torque on the front wheels is transmitted to the rear wheels. A direction selector valve for selectively changing fluid paths is provided in the hydraulic pressure closed circuit in order to compensate for the change in direction in which hydraulic fluid is discharged by the first and second hydraulic pumps which takes place when the vehicle changes its running direction from the straight-ahead running direction to a reverse running direction, so that a four-wheel driving condition is realized, irrespective of the running direction of the vehicle.
Additionally, a power transmission device for a four-wheel drive vehicle of this type is disclosed in JPA-9-202152. In this power transmission device, a drive shaft adapted to interlock with front wheels which are main drive wheels for rotation and a driven shaft adapted to interlock with rear wheels which are sub-drive wheels are connected to each other via a multiple disc clutch, and a two-way clutch mechanism is provided on the driven shaft. The two-way clutch mechanism has a function to distribute the torque on the front wheels to the rear wheels so as to improve the running through performance of the vehicle by locking when the front wheels slip and the rotation speed of the front wheels exceeds the rotation speed of the rear wheels in either of cases where the vehicle runs straight ahead and where the vehicle runs backward, as well as a function to unlock so that the torque on the front wheels is not distributed to the rear wheels to thereby avoid an adverse effect to the operation of an ABS (anti-lock brake system) when the front wheels lock and the rotation speed of the front wheels become lower than the rotation speed of the rear wheels.
The power transmission device described in the aforesaid JP-A-3-224830 requires two hydraulic pumps and therefore, the device has a drawback that the number of components used, the man hours involved in assembling those components, the weight of the device and the production cost thereof are increased.
In addition, the power transmission device described in the aforesaid JP-A-9-202152 has the two-way clutch mechanism provided on the driven shaft for transmitting the torque on the front wheels to the rear wheels and therefore, the torque is directly transmitted via the two-way clutch mechanism. This requires the use of a two-way clutch mechanism that has a large torque transmission capacity and which is expensive, causing problems that the power transmission device becomes larger and that the production cost is increased.
The invention was made in view of the above situations, and an object thereof is to provide a power transmission device for a four-wheel drive vehicle which enables the miniaturization thereof and cost reduction therefor by reducing the number of hydraulic pumps, as well as the torque transmission capacity of the two-way clutch mechanism.
With a view to attaining the above object, according to a first aspect of the invention, there is provided a power transmission device for a four-wheel drive vehicle comprising main drive wheels to which the torque of an engine is directly transmitted and sub-drive wheels to which a part of the torque of the engine is indirectly transmitted via a drive shaft, a hydraulic clutch and a driven shaft, wherein the hydraulic clutch locks under hydraulic pressure generated by a hydraulic pump driven by the drive shaft via a two-way clutch mechanism, the power transmission device being characterized in that, the two-way clutch locks when the rotation speed of the drive shaft exceeds the rotation speed of the driven shaft, irrespective of the rotating direction of the drive shaft, so as to drive the hydraulic pump.
According to the above construction, the two-way clutch mechanism is put in a non-locking condition at the time of straight-ahead constant speed running operation of the vehicle where the rotation speed of the main drive wheels coincides with the rotation speed of the sub-drive wheels and also at the time of braking operation of the vehicle which is running straight ahead where the rotation speed of the main drive wheels lowers below the rotation speed of the sub-drive wheels. As a result of this, the hydraulic pump is disengaged from the driving shaft and outputs no hydraulic pressure, and therefore, the hydraulic clutch is put in a non-locking condition, whereby the vehicle is maintained in a two-wheel drive condition.
On the other hand, the two-way clutch mechanism is put in a locking condition at the time of straight-ahead start-up and accelerating operations of the vehicle where the rotation speed of the main drive wheels exceeds the rotation speed of the sub-drive wheels, and therefore, the hydraulic pump is driven by the drive shaft to output a hydraulic pressure, and the hydraulic clutch is put in a locking condition, whereby the vehicle is switched over to a four-wheel drive condition.
Although the rotating direction of the respective components of the power transmission device when the vehicle runs straight backward becomes opposite to that when the vehicle runs straight ahead, the two-way clutch mechanism locks when the rotation speed of the drive shaft exceeds that of the driven shaft, irrespective of the rotating direction of the drive shaft, and therefore, as in the case of straight-ahead running operation, the two-way clutch mechanism is put in the non-locking condition at the time of straight-backward constant speed running operation of the vehicle and also at the time of braking operation of the vehicle which runs straight backward, whereby the vehicle can be maintained in the two-wheel drive condition, whereas the two-way clutch mechanism is put in the locking condition at the time of straight-backward start-up and accelerating operations of the vehicle, whereby the vehicle can be switched over to the four-wheel drive condition.
Then, there being no chance of the torque which is transmitted from the main drive wheels to the sub-drive wheels directly acting on the two-way clutch mechanism, only a minute torque for driving the hydraulic pump acts on the two-way clutch mechanism, and therefore, the torque transmission capacity of the two-way clutch mechanism can be small, so that the size and production cost of the same mechanism can be attempted at being decreased. Additionally, only one hydraulic pump is required, and therefore, this can contribute to the reduction of the number of components used in the two-way clutch mechanism, the man hours involved in assembling the components into the same mechanism, the weight of the mechanism and the production coast thereof. Moreover, the hydraulic pump is designed to be actuated only when the rotation speed of the main drive wheels exceeds the rotation speed of the sub-drive wheels, and therefore, energy needed to drive the hydraulic pump can be maintained as small as possible, whereby the fuel consumption of the engine can be reduced to save on fuel used.
According to a second aspect of the invention, there is provided a power transmission device for a four-wheel drive vehicle as set forth in the first aspect above, wherein the two-way clutch mechanism comprises a first clutch element connected to the drive shaft, a second clutch element connected to the hydraulic pump and a direction selector valve connected to the driven shaft via a friction member in such a manner as to allow a slip.
According to the above construction, the direction selector element is connected to the driven shaft via the friction member in such a manner as to allow a slip, and therefore, even when the difference in rotation speed between the main drive wheels and the sub-drive wheels, a risk of an excessive load being applied to the direction selector element can be eliminated, thereby making it possible to securely maintain the durability of the two-way clutch mechanism.
According to a third aspect of the invention, there is provided a power transmission device for a four-wheel drive vehicle as set forth in the second aspect above, wherein a speed increasing unit is interposed between the direction selector element of the two-way clutch mechanism and the driven shaft for increasing the rotation speed of the driven shaft for transmission to the direction selector element.
According to the above construction, the rotation speed of the driven shaft is increased by the speed increasing unit for transmission to the direction selector element, and therefore, it is not until the rotation speed of the main drive wheels exceeds the rotation speed of the sub-drive wheels beyond the speed increasing rate of the speed increasing unit that the two-way clutch locks. Consequently, even if a slight difference in rotation speed between the main drive wheels and the sub-drive wheels occurs when the vehicle makes a tight turn or is driven to be parked in a parking slot, since the two-way clutch mechanism does not lock, a tight corner braking phenomenon can be avoided, whereby the vehicle is prevented from being put in the four-wheel drive condition unnecessarily, thereby making it possible to reduce the fuel consumption of the engine to save on fuel used.
According to a fourth aspect of the invention, there is provided a power transmission device for a four-wheel drive vehicle as set forth in the first aspect above, wherein the two-way clutch mechanism is disposed coaxially in the interior of the hydraulic pump.
According to the above construction, since the two-way clutch is disposed coaxially in the interior of the hydraulic pump, the axial dimension of the power transmission device can be reduced.